Terahertz (THz) imaging and spectroscopy has emerged as a powerful tool for analysis of biological specimens or for studying and controlling low-energy excitations in solid-state systems [1]. One major limitation of THz spectroscopy is, however, the spatial resolution that is limited by diffraction to about 100-1000 µm, making it impossible to extract intrinsic, local material characteristics of nanoscale Graphene or other 2D-material structures or devices.

Scattering-type scanning near-field optical microscopy (s-SNOM) bypasses the diffraction limit, enabling optical measurements with extreme sub-wavelength spatial resolution of below 20 nm [2]. THz time-domain s-SNOM spectroscopy of a semiconductor static random access memory (SRAM) sample enables quantitative analysis of free carrier concentration and scattering rates of nanoscale device structures at unprecedented surface sensitivity. Similarly, THz s-SNOM measurements at single frequencies enabled highlighting highly conductive nanostructures in transistor devices [3]. Using a similar experimental concept to generate a local photo-current by THz radiation allows for studying nanoscale conductivity in a biased graphene device [4] or to characterize carrier scattering in a graphene sheet that has been encapsulated in hexagonal boron nitride (h-BN) and placed on a split metallic film [5].

Using an SRAM sample as a method to calibrate the amplitude contrast of the s-SNOM THz measurement with known free carrier concentrations the concept is extended to graphene. Single to few layer graphene on h-BN is imaged with a broadband THz pulse and reveals changing amplitude contrast independently of layer number (Fig. (a), (b), zoom in (d)). In a corresponding s-SNOM measurement carried out at 10 µm an inverted contrast can be observed (Fig. (c)). This contrast inversion is a direct consequence of the scattering frequency dependent free carrier response in graphene [6].

Extending the already established concepts of mid-IR s-SNOM imaging and spectroscopy of 2D materials [7] to the THz frequency range a plethora of fundamental and applied new insights into graphene and other 2D device structures can be expected from THz near-field nanoscopy measurements.

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